1. Field of the Invention
The present invention concerns a method of measuring the excess length of optical fibers in an optical fiber cable.
2. Description of the Prior Art
In the design of an optical fiber cable, the fiber strain is a limiting factor on the tensile load capacity. Therefore, a small amount of excess optical fiber, typically about 0.07 percent, is included to ensure that the fibers are not under tension in the manufactured cable. However, too much fiber results in an excess loss from microbending. In the prior art, to ensure that the manufacturing tolerances are not resulting in an excess amount of fiber being included in the cable, a small percentage of the manufactured cables is first measured for length and then split open, with the optical fibers being removed. The length of the fibers in an uncompressed, straightened state is then measured. However, this is a destructive testing technique and hence requires that a finished optical fiber cable must be sacrificed.
Clearly, a nondestructive method of measuring excess optical fiber in a cable is very desirable. Methods are known by which the tensile strain on an optical fiber can be measured. For example, a pulse-delay method may be utilized which determines the transit time of an optical pulse before and after a strain is applied. It is also known to measure strain in an optical fiber by the use of the phase change of an optical signal that propagates through the fiber. In that technique, an optical source is modulated, and the optical signal is propagated through the fiber. The phase difference in the signal at the other end of the fiber is compared to an internal reference. This technique has also been used to measure compressive strain in an optical fiber; see, for example, "Single-Ended On-Line Extrusion-Coating Induced Strain Measurements in Optical Fibers Using Phase Delay," by R. Kashyap et al in Proceedings Of The Third International Conference On Integrated Optics And Optical Communication (San Francisco), April 1981. However, that technique is suitable only to the extent that the fiber can withstand a compression without buckling. It is suitable, for example, in a manufacturing operation wherein a relatively thick coating is applied to a fiber that helps prevent buckling during the manufacturing process, which can occur, for example, when the coating material shrinks longitudinally on the fiber. However, in the cabling process, once the fiber buckles there is substantially no further compression on the fiber, and hence the above technique yields no useful information on the degree of buckling of a fiber in a cable.